In a direct injection internal combustion engine, a fuel injector is provided to deliver a charge of fuel to a combustion chamber prior to ignition. Typically, the fuel injector is mounted in a cylinder head with respect to the combustion chamber such that its tip protrudes slightly into the chamber in order to deliver a charge of fuel into the chamber.
One type of fuel injector that is particularly suited for use in a direct injection engine is a so-called piezoelectric injector. A piezoelectric injector 12 and its associated control system 14 are shown schematically in FIG. 1.
The piezoelectric injector 12 includes a piezoelectric actuator 16 that is operable to control the position of an injector valve needle 17 relative to a valve needle seat 18. The piezoelectric actuator 16 includes a stack 19 of piezoelectric elements, having the electrical characteristics of a capacitor. The stack 19 of piezoelectric elements expands and contracts in dependence on a differential voltage applied across the terminals of the actuator to charge or discharge the actuator. The expansion and contraction of the piezoelectric elements is used to vary the axial position, or ‘lift’, of the valve needle 17 relative to the valve needle seat 18.
By application of an appropriate voltage differential across the actuator 16, an injection event is initiated, whereby the valve needle 17 is caused to disengage the valve seat 18, causing fuel to be delivered into an associated combustion chamber (not shown) through a set of nozzle outlets 20. Similarly, by application of an appropriate voltage differential across the actuator 16, the valve needle is caused to engage the valve seat 18, to prevent fuel delivery through the outlets 20 and terminate the injection event.
The piezoelectric injector 12 is controlled by an injector control unit 22 (ICU) that forms an integral part of an engine control unit 24 (ECU). The ICU 22 typically comprises a microprocessor 26 and memory 28. The ECU 24 also comprises an injector drive circuit 30, to which the piezoelectric injector 12 is connected by way of first and second power supply leads 31, 32.
Piezoelectric injectors are typically grouped together in banks. As described in EP1400676, each bank of piezoelectric injectors has its own drive circuit for controlling operation of the piezoelectric injectors. The use of these drive circuits enables the voltage applied across the piezoelectric fuel injectors, to be controlled dynamically. This may be achieved by using two storage capacitors that are alternately connected to the injector bank.
One of the storage capacitors is connected to the injector bank during a charge phase when a charge current flows through the injector bank to charge an injector, thereby initiating an injection event in a ‘charge-to-inject’ fuel injector, or terminating an injection event in a ‘discharge-to-inject’ fuel injector. The other storage capacitor is connected to the injector bank during a discharge phase, to discharge the injectors, thereby terminating the injection event in a charge-to-inject fuel injector, or initiating an injection event in a discharge-to-inject fuel injector. The expressions “charging the injectors” and “discharging the injectors” are used for convenience and refer to the processes of charging and discharging, respectively, the piezoelectric actuators of the fuel injectors.
Like any circuit, faults may occur in a drive circuit. In safety critical systems, such as diesel engine fuel injection systems, a fault in the drive circuit may lead to a failure of the injection system, which could consequentially result in a catastrophic failure of the engine. Such faults include short circuit faults and open circuit faults in the piezoelectric actuators of the fuel injectors. A typical short circuit fault that may occur is a short circuit between the terminals of the piezoelectric actuator; otherwise referred to as a ‘stack terminal’ short circuit.
Diagnostic techniques for detecting short circuit, and open circuit faults in the piezoelectric actuators are disclosed in applicant's co-pending patent applications EP 06251881.6, EP 06253619.8, EP 06256140.2, and EP 07252534.8, the contents of each document being incorporated herein by reference. However, there is a need to develop further diagnostic techniques in order to detect faults that might otherwise not be detected by these techniques.